In the field of commercial dishwashers—be they conveyor-type dishwashers having several liquid tanks or hood-type dishwashers or other stationery machines with only one liquid tank—it is necessary for fresh water to be introduced into the system at least at one point and for used washing water to be discharged from the system at least at one point. Since the fresh water is usually drawn from the public drinking water system or the like, it is at a comparatively low temperature which is not suitable for all dishwashing zones or dishwashing processes. Therefore, final rinsing can of course be performed with comparatively cold fresh water; however, at the latest when the water is intended to be used as cleaning water, for example, in the next dishwashing section of the washing water cascade in the case of a conveyor-type dishwasher or at a corresponding program point in the case of a batch dishwasher and, for example, with a detergent or the like admixed, low washing water temperatures of this kind are no longer sufficient.
If the washing water is now heated to the required temperature as is customary, the question arises of whether the heat remaining in the washing water after said washing water is used, for example by partial or complete replacement of the used washing water with fresh water, can be used.
In the case of commercial hood-type dishwashers, approximately 50% of the supplied thermal energy is discharged as lost heat in the form of heated waste water.
The remainder of the supplied thermal energy remains in the washware or is lost as a result of vapor discharge or the like.
It is known in the art to use the thermal energy in the waste water, before said waste water is discharged to the waste water system, by means of a heat exchanger in such a way that this heat in the outflowing waste water—physically separately from the fresh water—is at least partially transmitted to the supplied fresh water by means of a heat exchanger. The conventional solutions now have the disadvantage that they sometimes do not function reliably enough and, in particular, the waste water remains at a comparatively high temperature when introduced into the waste water system, as a result of which less thermal energy is emitted to the supplied fresh water. This is the result of, for example, heat exchangers which are composed of plastic materials often being used, these having a low thermal conductivity on account of the material used. In addition, plate-type heat exchangers or the like which are used can become clogged if the washing water is heavily soiled (food residues), wherein these food residues collect between the plates of the plate-type heat exchanger and block the liquid channel.
An excessively high waste water temperature is also disadvantageous in that it is not possible to comply with any existing local standards. For example, the US “Uniform Plumbing Code” specifies a maximum waste water temperature of 140° Fahrenheit (60° C.) wherein, if this auxiliary limit cannot be complied with, cold fresh water is often supplied to the waste water in order to comply with the required maximum waste water value.
The invention is therefore based on the object of specifying a dishwasher having a corresponding heat recovery device which operates more reliably than conventional solutions and at the same time has a good energy yield and can be produced in a cost-effective manner.